Recently, various flat panel displays have been developed, to substitute cathode ray tube (CRT) displays because the CRT displays are relatively heavy and bulky. Among the flat panel displays, a light emitting display (LED) is notable because it has high emission efficiency, high brightness, wide view angle, and fast response time.
The light emitting display comprises a plurality of light emitting devices, wherein each light emitting device has a structure in which an emission layer is placed between a cathode electrode and an anode electrode. Here, an electron and a hole are injected into the emission layer and recombined to create an exciton. Light is emitted when the exciton falls to a lower energy level.
Such a light emitting display is classified into an inorganic light emitting display comprising an inorganic emission layer, and an organic light emitting display comprising an organic emission layer.
FIG. 1 is a circuit diagram of a pixel provided in a conventional light emitting display. Referring to FIG. 1, the pixel comprises an organic light emitting device OLED, a driving transistor M2, a capacitor Cst, a switching transistor M1. Further, the pixel is connected to a scan line Sn, a data line Dm, a pixel power line Vdd, and a second power supply line Vss. The second power supply line Vss is a voltage lower that the first voltage supply, for example, a ground voltage. Here, the scan line Sn is arranged in a row direction, and the data line Dm and the pixel power line Vdd are arranged in a column direction. For reference, n is an arbitrary integer between 1 and N, and m is an arbitrary integer between 1 and M.
The switching transistor M1 comprises a source electrode connected to the data line Dm, a drain electrode connected to a first node A, and a gate electrode connected to the scan line Sn.
The driving transistor M2 comprises a source electrode connected to the pixel power line Vdd, a drain electrode connected to the organic light emitting device OLED, and a gate electrode connected to the first node A. Here, the driving transistor M2 supplies current to the organic light emitting device OLED in response to a signal inputted to its gate electrode, thereby allowing the organic light emitting device to emit light. Further, the intensity of the current flowing in the driving transistor M2 is controlled by a data signal transmitted through the data line Dm and switching transistor M1.
The capacitor Cst comprises a first electrode connected to the source electrode of the driving transistor M2, and a second electrode connected to the first node A. Here, the capacitor Cst maintains voltage applied between the source and gate electrodes of the driving transistor M2 in response to the data signal, for a predetermined period.
With this configuration, when the switching transistor M1 is turned on in response to the scan signal transmitted to the gate electrode of the switching transistor M1, the capacitor Cst is charged with a voltage corresponding to the data signal, and then the voltage charged in the capacitor Cst is applied to the gate electrode of the driving transistor M2. Hence, the current flows in the driving transistor M2, thereby allowing the organic light emitting device OLED to emit light.
At this time, the current supplied from the driving transistor M2 to the organic light emitting device OLED is calculated by the following equation.
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                                    β              2                        ⁢                                          (                                  Vdd                  -                  Vdata                  -                  Vth                                )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            where IOLED is a current flowing in the organic light emitting device OLED; Vgs is a voltage applied between the source and gate electrodes of the driving transistor M2; Vth is a threshold voltage of the driving transistor M2, Vdata is a voltage corresponding to the data signal; and β is a gain factor of the driving transistor M2.
Referring to the equation 1, the current IOLED flowing in the organic light emitting device OLED varies depending on the threshold voltage of the driving transistor M2.
However, when the conventional light emitting display is fabricated, deviation arises in the threshold voltage of the driving transistor M2. Thus, the deviation in the threshold voltage of the driving transistor M2 causes in consistencies in the current flowing in the organic light emitting device OLED to be not uniform, thereby deteriorating the uniformity of the brightness of the display device.
Further, the pixel power line Vdd connected to each pixel and supplying pixel power is connected to a first power line (not shown) and supplies the pixel power. In this case, voltage drop arises in the first power supplied from the pixel power line Vdd to the first power line. As the length of the first power line increases, the pixel power line Vdd connected thereto increases in number, thereby causing the voltage drop to get larger.
Particularly, for a large screen of the flat panel display, the voltage drop in the first power line increases further.